Known EGR apparatuses are divided into two types. In the first type of EGR apparatus, the exhaust gas is recirculated into the intake system of the engine in a position located upstream of the throttle valve. In the second type of EGR apparatus, the exhaust gas is recirculated into the intake system in a position located downstream of the throttle valve. A characteristic of the first type EGR apparatus is that said EGR apparatus has a proportion of recirculated gas, i.e., the ratio of the amount of the exhaust gas being recirculated to the amount of the intake air to the engine, which is always constant irrespective of the degree of the throttle opening. On the other hand, the second type EGR apparatus has such a characteristic that the larger the throttle opening, the lower will be the recirculated gas proportion.
In these EGR apparatuses it is desired to recirculate a sufficient amount of EGR gas for reducing to a great extent the amount of nitrogen oxides (NO.sub.x) emission, due to the strict regulations directed against exhaust gas pollution from engines which is undoubtedly a great public nuisance.
However, the above-mentioned first type EGR apparatus suffers from the following shortcoming. Since its proportion of recirculated gas remains constant during a progression from a light load operation (in which the throttle opening is small) to a heavy load operation (in which the throttle opening is large), if the proportion of EGR is increased during the light load operation, too much exhaust gas is recirculated during the heavy load operation, which causes adverse effects to the heavy load operation, for example, to the accelerating operation.
The above-mentioned second type EGR apparatus has the following shortcoming. Since its proportion of recirculated gas decreases from a light load operation to a heavy load operation, if said proportion is increased during the light load operation, too little exhaust gas can be recirculated during the heavy load operation; therefore, effective elimination of the NO.sub.x components during the heavy load operation cannot be expected.